Cosmetic brushes have traditionally used horse hair, goat hair and other animal hair, and these animal-hair bristles are believed to feel pleasant on the skin, or in other words feel good when used, and also offer good loading property (ease of picking up and holding) with respect to powder and other cosmetic materials, as well as good transfer property in transferring cosmetic materials onto the skin. Accordingly, various horse-hair cosmetic brushes are available on the market. The photograph in FIG. 2 is an image of the surface of a horse's axillary tail hair taken by a laser microscope (VK-Analyzer VK-8710 by Keyence), and clearly many projections are found on the surface of the horse hair. These many projections are believed to explain the good loading property and transfer property of horse hair with respect to powder and other cosmetic materials.
Having the aforementioned advantages, however, animal hair has drawbacks, such as being a natural resource and therefore limited in supply. For this reason, cosmetic brush bristle materials made of synthetic fibers have been proposed in recent years as substitutes for animal hair.
For example, Patent Literature 1 proposes a cosmetic brush bristle material having concaves on the surface. One hundred parts by weight of polybutylene terephthalate (PBT) are mixed with 5 to 15 parts by weight of silica, talc, silver zeolite, or other inorganic powder whose average particle size is 0.5 to 1.0 μm, after which the mixture is melted and spun into a filament and the obtained filament is drawn to 5 to 6 times longer to cause the aforementioned inorganic powder to sink and thereby form concaves on the surface. A bundle of such filaments is cut to a specific length and one end of the obtained fiber bundle is soaked in an alkali solution to enlarge the aforementioned concaves, while the other end is melted to reduce weight and formed into a tapered shape, to produce the proposed cosmetic brush bristle material (refer to Patent Literature 1). It is disclosed that a cosmetic brush bristle material using the aforementioned filament having concaves formed on it provides loading property and transfer property equivalent to those of animal hair because the concaves formed on the filament surface and enlarged by alkali treatment act like cuticles of animal hair.
Patent Literature 2 proposes a brush bristle material having surface irregularities. It is disclosed that, to taper the tip of this brush bristle material, polytrimethylene terephthalate (PTT) is melted and spun into a filament and the obtained filament is drawn to 5 to 6 times longer, and one end of a bundle of such filaments is soaked by approx. 10 to 20 mm in the length direction in an alkali treatment solution containing amine catalyst and treated for 1 to 2 hours at 110 to 130° C., to form surface irregularities of 1 to 20 μm at the tapered tip of the filament as a result of alkali treatment, without having to blend an inorganic powder (refer to Patent Literature 2). With the cosmetic brush bristle materials in Examples 1 and 2, the fineness of the PTT filament is 80 dtex for the former and 100 dtex for the latter.